Halogenated dibenzofurans are found worldwide as environmental pollutants. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the degree and position of halogenation. This work has established that 2,3,7,8-tetrachlorodibenzofuran (TCDF), an extremely toxic isomer, is excreted only after metabolism and toxicity is inversely related to metabolic capability. The metabolites of TCDF produced by the rat are being characterized. There appear to be threshold body burden for toxicity. The distribution to the fetus was examined after maternal exposure. The role of body composition on the disposition of 2,3,7,8-tetrachlorodibenzodioxin (TCDD), the most toxic man-made compound known, is being examined in congenic mouse strains which are sensitive or resistant to TCDD toxicity. Absorption of octachlorodibenzo-p-dioxin (OCDD) from the gastrointestinal tract and the effect of dose on absorption is being studied in the rat.